The peripheral chemoreceptors (carotid bodies) detect changes in blood oxygen and / or pH whereas central chemoreceptors detect changes in brain extracellular fluid pH. When activated, these receptors produce a large spectrum of effects that include the stimulation of breathing, a rise in blood pressure and sympathetic nerve activity, hormonal changes and behavioral arousal. These physiological responses are important for homeostasis and of considerable clinical interest. For instance, acute and chronic activation of the sympathetic vasomotor outflow and sleep fragmentation are the two most deleterious consequences of obstructive sleep apnea (OSA), a common disease characterized by frequent bouts of asphyxia during the night that cause repetitive chemoreceptor stimulation (69). In the congenital central hypoventilation syndrome, the reverse problem seems to occur: asphyxia does not produce awakening from sleep and autonomic reflexes are depressed (4;15;87). In this renewal application we propose to explore how chemoreceptor stimulation activates the cardiovascular system, the hypothalamo-pituitary axis and causes arousal. We will test the hypothesis that all of these effects could be mediated by the activation of the C1 adrenergic neurons of the rostral medulla. To address this theory, we have developed new techniques that will enable us to stimulate or inhibit the C1 cells selectively in both anesthetized and unanesthetized rats. This new technology consists of targeting light or ligand-activated receptors selectively to the C1 cells using lentiviruses and the synthetic dopamine beta-hydroxylase promoter PRSx8. By combining this methodology with mouse genetics and a repertoire of more traditional neuroanatomical and neurophysiological methods, we propose to pursue the following four specific aims: In Aim 1 we will determine whether the selective stimulation or inhibition of the C1 cells alters respiration, blood pressure and sympathetic nerve activity in the presence or absence of anesthesia in rats. In Aim 2 we will determine whether chemoreceptor stimulation activates a major wake-promoting nucleus, the locus coeruleus, via the C1 neurons and whether C1 cell stimulation produces arousal from sleep. In Aim 3 we will determine whether chemoreceptor stimulation activates the hypothalamo-pituitary axis also via the C1 neurons. In Aim 4 we will determine whether C1 neurons are directly activated by brain acidification or whether these cells derive their pH sensitivity from other neurons, in particular from the central chemoreceptors located in the retrotrapezoid nucleus. PUBLIC HEALTH RELEVANCE: Obstructive sleep apnea (OSA) is a very prevalent chronic disease that causes disorganized sleep, acute hypertensive and tachycardiac episodes during sleep, chronic hypertension and exacerbation of the metabolic syndrome. OSA increases cardiovascular morbidity and mortality and causes driving fatalities due to day-time somnolence. In this project we seek to understand why asphyxia, such as occurs during OSA, raises blood pressure, stimulates breathing efforts and promotes awakenings. We postulate that all these effects could be either triggered or at least facilitated by the activation of a group of brainstem neurons called the C1 cells which are vigorously activated by hypoxia and hypercapnia and regulate the autonomic nervous system and stress-related autonomic responses.